WASHINGTON — Speculation about Mitt Romney’s running mate is at fever pitch in the U.S. capital now that the former Massachusetts governor is all but assured the Republican presidential nomination.

In the corridors of power on Capitol Hill, at swank Georgetown dinner parties, in K Street bars and on the talk-show circuit, it’s the subject of intense debate: Will it be Condoleezza Rice? Marco Rubio? Jeb Bush? Paul Ryan? Chris Christie?

Rubio, the Cuban-American U.S. senator from Florida, did little to silence the buzz surrounding his name on Sunday when he appeared on CNN’s “State of the Union.”